Seeker's Armguard

Seeker's Armguard is way to cost efficient for how much it actually costs. The item at no stacks is 95.3% cost efficient. The item at max stacks is 147.5% cost efficient. That's more cost effective than Morellonomicon. And for killing 30 minions (about 5 waves (150 seconds) not including siege(150 seconds)), you have a Warden's Mail that has a bonus 40 AP.

I would say the best thing to do here is to reduce the starting armor of the item to 20 so that it would be more balanced at full stacks. Also, Hex Drinker is around as cost efficient as Seeker's Armguard but only every 90 seconds. There should also be an item that grants a bit of armor and AD for AD based Champions.